Editing Postcodes in the United Kingdom (section)

==Formatting==
===Overview===
The postcodes are alphanumeric, and are (possibly uniquely) variable in length: ranging from six to eight characters (including a space). Each postcode is divided into two parts separated by a single space: the ''outward code'' and the ''inward code'' respectively. The ''outward code'' includes the ''postcode area'' and the ''postcode district'', respectively. The ''inward code'' includes the ''postcode sector'' and the ''postcode unit'' respectively. Examples of postcodes are "SW1W 0NY", "PO16 7GZ", "GU16 7HF", and "L1 8JQ".<ref>{{Cite web |title=SW1W 0NY Postcode - Greater London, England, UK |url=https://trackpostcode.uk/postcode/sw1w/0ny/ |access-date=2024-10-28 |website=trackpostcode.uk}}</ref>

{| class="wikitable" style="text-align: center; margin: 0 auto;"
!style="background:#87cefa; color: black;" colspan=4| POSTCODE
|- style="background:#ff4500; color: white;"
|colspan=2| Outward code
|colspan=2| Inward code
|- style="background:#ff4500; color: white;"
| Area || District || Sector || Unit
|- style="background:#0000cd; color: white;"
| SW || 1W || 0 || NY
|}

===Outward code===
The outward code is the part of the postcode before the single space in the middle. It is between two and four characters long. Examples of outward codes are "L1", "W1A", "RH1", "RH10" or "SE1P". A few outward codes are non-geographic, not divulging where mail is to be sent.  These postcodes cannot be used for navigation purposes.

====Postcode area====
The [[List of postcode areas in the United Kingdom|postcode area]] is part of the outward code. The postcode area is either one or two characters long and is alphabetical, with there being 121 such areas. Examples of postcode areas are "L" for [[Liverpool]], "RH" for [[RH postcode area|Redhill]] and "EH" for [[Edinburgh]]. A postal area may cover a wide area, for example "RH" covers various settlements in eastern Surrey and north eastern West Sussex, and "BT" ([[Belfast]]) covers the whole of [[Northern Ireland]].

====Postcode district====
The [[List of postcode districts in the United Kingdom|postcode district]] is one digit, two digits or a digit followed by a letter.

===Inward code===
The inward code is the part of the postcode after the single space in the middle. It is three characters long. The inward code assists in the delivery of post within a postal district. Examples of inward codes are "0NY", "7GZ", "7HF", or "8JQ".<ref name="Postcodes & Addresses Explained" />

====Postcode sector====
The postcode sector is made up of a single digit (the first character of the inward code). Most postcode areas do not use all of the sectors 0–9 in order to allow for the possibility of more sectors being added in the face of new development. Rather, in the initial allocation of postcodes, neighbouring postcode districts were often assigned to contain the ten sectors between them. For example, across the three postcode districts [[BS postcode area|BS6-8]] (which are next to each other in north west [[Bristol]]), sectors 1-4 were assigned to BS8, sectors 5-7 were assigned to BS6, and sectors 8-9 and 0 were assigned to BS7 (more recent changes have resulted in all three of those areas now having a sector 9).<ref>{{cite web |last1=Bell |first1=Chris |title=BS Postcode Area Bristol |url=https://www.doogal.co.uk/UKPostcodes?Search=BS |website=Doogal |access-date=1 September 2024}}</ref>

====Postcode unit====
The postcode unit is two characters added to the end of the postcode sector. A postcode unit generally represents a street, part of a street, a single address, a group of properties, a single property, a sub-section of the property, an individual organisation or (for instance [[Driver and Vehicle Licensing Agency]]) a subsection of the organisation. The level of discrimination is often based on the amount of mail received by the premises or business.

===Validation===
The format is as follows, where '''A''' signifies a letter and '''9''' a digit:

{| class="wikitable"
|-
! Format !! Coverage !! Example
|-
! style="text-align:left; white-space:nowrap;" | AA9 9AA
| rowspan="2" |All postcodes except those listed below
| style="white-space:nowrap;" | CR2 6XH
|-
! style="text-align:left; white-space:nowrap;" | AA99 9AA
| style="white-space:nowrap;" | DN55 1PT
|-
! style="text-align:left; white-space:nowrap;" | A9 9AA
| rowspan="2" |B, E, G, L, M, N, S, W
| style="white-space:nowrap;" | M1 1AE
|-
! style="text-align:left; white-space:nowrap;" | A99 9AA
| style="white-space:nowrap;" | B33 8TH
|-
! style="text-align:left; white-space:nowrap;" | A9A 9AA
| London postcode districts: E1, N1, W1
| style="white-space:nowrap;" | W1A 0AX
|-
! style="text-align:left; white-space:nowrap;" | AA9A 9AA
| London postcodes: WC postcode area; and districts EC1–EC4, NW1W, SE1P, SW1
| style="white-space:nowrap;" | EC1A 1BB
|}

Notes:
* As all formats end with 9AA, the first part of a postcode can easily be extracted by ignoring the last three characters.
* Areas with only single-digit districts: BL, BR, FY, HA, HD, HG, HR, HS, HX, JE, LD, SM, SR, WC, WN, ZE (although WC is always subdivided by a further letter, e.g. WC1A)
* Areas with only double-digit districts: AB, LL, SO (for AB this arose from decoding of the original five districts AB1-AB5 by adding a second digit, to enable additional postcodes to become available, thus AB1 was divided into AB10-AB16).
* Areas with a district '0' (zero): BL, BS, CM, CR, FY, HA, PR, SL, SS (BS is the only area to have both a district 0 and a district 10)
* The following central London single-digit districts have been further divided by inserting a letter after the digit and before the space: EC1–EC4 (but not EC50), SW1, W1, WC1, WC2 and parts of E1 (E1W), N1 (N1C and N1P), NW1 (NW1W) and SE1 (SE1P).
* The letters ''Q'', ''V'' and ''X'' are not used in the first position.
* The letters ''I'', ''J'' and ''Z'' are not used in the second position.
* The only letters to appear in the third position are ''A'', ''B, C, D, E, F, G, H, J, K, P, S, T, U'' and ''W'' when the structure starts with A9A.
* The only letters to appear in the fourth position are ''A, B, E, H, M, N, P, R, V, W, X'' and ''Y'' when the structure starts with AA9A.
* The final two letters do not use ''C, I, K, M, O'' or ''V'', so as not to resemble digits or each other when hand-written.
* Postcode districts are one of ten digits: 0 to 9, with 0 only used once 9 has been used in a post town, save for Croydon (see above).
* Postcode sectors can also be one of ten digits: 0 to 9, though in some postcode areas the 0 is the beginning of the sequence (for example in LE), while in other areas it is the end of the sequence (i.e. 10, as in CV).

A postcode can be validated against a table of all 1.7&nbsp;million postcodes in [https://www.ordnancesurvey.co.uk/products/code-point-open Code-Point Open]. The full delivery address including postcode can be validated against the [[Royal Mail]] [[Postcode Address File]] (PAF), which lists 29&nbsp;million valid delivery addresses,<ref name=bbc_50>[http://news.bbc.co.uk/1/hi/uk/7803267.stm Postcodes to celebrate 50th year] [[BBC News]], 30 December 2008</ref> constituting most (but not all) addresses in the UK.<ref>{{cite web|
title=Falsehoods programmers believe about addresses|
url=http://www.mjt.me.uk/posts/falsehoods-programmers-believe-about-addresses/|
author=Michael Tandy|website=Mjt.me.uk|access-date=8 October 2017}}</ref> A [[regular expression]] for validating UK postcodes is specified in the [[British Standards]] document BS 7666.<ref>{{cite web|url=https://assets.publishing.service.gov.uk/media/632b07338fa8f53cb77ef6b8/WS02_LRS_Web_Services_Interface_Specification_v6.4.pdf|title=LRS Web Services Interface Specification|publisher=[[Education and Skills Funding Agency]]|date=September 2022|at=Appendix D|access-date=17 July 2024}}</ref>

===Post towns===
All or part of one or more postcode districts are grouped into [[post town]]s.<ref name="Guide">{{Cite book |title=Address Management Guide |edition=5th |publisher=Royal Mail Group plc |date=March 2007 |url=http://www.royalmail.com/marketing-services/address-management-unit/address-data-products/address-management-guide |access-date=25 October 2012}}</ref> Larger post towns may use more than one postcode district, for example Crawley uses RH10 and RH11. In a minority of cases, a single number can cover two or more post towns – for example, the WN8 district includes Wigan and Skelmersdale post towns; and the GL17 district contains five post towns.